Projection device and method for holographic reconstruction of scenes

ABSTRACT

A holographic reconstruction of scenes includes a light modulator, an imaging system with at least two imaging means and an illumination device with sufficient coherent light for illumination of hologram coded in the light modulator. The at least two imaging means are arranged such that a first imaging means is provided for the magnified imaging of the light modulator on a second imaging means. The second imaging means is provided for imaging of a plane of a spatial frequency spectrum of the light modulator in a viewing plane at least one viewing window. The viewing window corresponds to a diffraction order of the spatial frequency spectrum.

CROSS-REFERENCE TO RELATED APPLICATIONS

This is a divisional of U.S. application Ser. No. 11/914,278, filed Nov.13, 2007, which is the US national stage of International ApplicationNo. PCT/DE2006/000896, filed May 12, 2006, which claims priority toGerman Application No. DE 10 2005 023 743.6, filed on May 13, 2005, theentire contents of each of which being fully incorporated herein byreference.

BACKGROUND OF THE INVENTION

The present invention relates to a projection device for the holographicreconstruction of scenes, said device comprising a spatial lightmodulator, an imaging system with at least two imaging means and anillumination system with at least one light source to generatesufficiently coherent light for the illumination of a hologram encodedon the light modulator. This invention further relates to a method forthe holographic reconstruction of scenes.

Known 3D displays or 3D projection devices and methods usually takeadvantage of the stereo effect, where the light which generates thestereo impression is reflected on or emitted from a plane. However, inholography the light which is emitted by the hologram interferes in theobject points of the scene, from where it propagates naturally.Holographic representations are object substitutions. In contrast, anyforms of stereoscopic representations of unmoving (stills) or movingscenes do not represent object substitutions. They rather provide twoplane images, one for the left and one for the right eye, where saidimages correspond with the two eye positions. The three-dimensionaleffect is created by the parallax in the two pictures. In a holographicrepresentation, the problems known in conjunction with stereoscopy, suchas fatigue, eyestrain and headache, do not occur, because there isgenerally no difference between viewing a real scene and aholographically reconstructed scene.

In holography it is generally distinguished between static and dynamicmethods. In static holography, photographic media are predominantly usedfor information storage. This means that a reference beam issuperimposed by a light beam which carries the object information suchto record an interference pattern on the photographic medium. Suchstatic object information is reconstructed with the help of a beamsimilar or identical to the reference beam. However, for example theentertainment industry or medical and military equipment manufacturershave been interested for a long time in a real-time representation ofmoving scenes using dynamic holography, because of the ideal spatialproperties of such reconstructions. In most cases, micro displays of thesame type as used in projection devices are employed. Micro displays canbe, for example, liquid crystal on silicon (LCoS) panels, transmissiveLCD panels or micro electro-mechanical systems (MEMS). Because theirdistance between the pixel centres, the pixel pitch, is small comparedto other displays, a relatively large diffraction angle is achieved. Amajor disadvantage of hitherto known dynamic holographic methods whichinvolve micro displays, however, is that the size of the reconstructionsor of the reconstructed scenes is greatly limited by the size of themicro displays. Micro displays and similar light modulators have a sizeof few inches and, despite the relatively small pitch, still adiffraction angle which is so small that viewing a scene with both eyesis hardly possible. A very small pitch of only 5 μm, for example,results in a diffraction angle of about 0.1 rad at a wavelength λ of 500nm (blue-green). At an observer distance of 50 cm, a lateral dimensionof 5 cm is achieved, which does not allow the scene to be viewed withboth eyes.

For a three-dimensional representation of dynamic holograms, typicallycomputer-generated holograms, holographic reconstruction devices takeadvantage of transmissive or reflective light modulators, such as TFT,LCoS, MEMS, DMD (digital micro-mirror device), OASLM (opticallyaddressed spatial light modulators), EASLM (electronically addressedspatial light modulators) and FLCD (ferro-electric liquid crystaldisplays) etc. Such light modulators can be of one- or two-dimensionaldesign. The reasons why reflective light modulators are used are aninexpensive manufacturing process, a large fill factor for great lightefficiency, short switching delays and only little light loss caused byabsorbance compared with transmissive displays. However, the smallerspatial dimensions must be put up with.

WO 03/060612 describes a reflective LC display with a resolution ofabout 12 μm and a reflectance of up to 90% for real-time colourreconstruction of holograms. The reconstruction is carried out using thecollimated light of one or multiple LEDs through a field lens. With thisresolution, viewing is only possible in a region which is just about 3cm wide at a distance of about 1 m, which is insufficient for thereconstructed scene to be viewed simultaneously with both eyes, i.e. ina three-dimensional way. Further, only relatively small objects can bereconstructed because of the small dimensions of the display.

WO 02/095503 discloses a holographic 3D projection device which uses aDMD chip for hologram reconstruction. However, despite the relativelyhigh resolution, great reflectance and low switching delays of the lightmodulator, this device also only allows scenes with a small size to bereconstructed and to be viewed in a very small region for the samereasons mentioned in conjunction with WO 03/060612. The reason for thisis again the small reconstruction space, which is defined by thedimensions of the light modulator and visibility region. Furthermore,DMD chips only partly suit holographic purposes due to their limitedcoherence.

WO 00/75699 discloses a holographic display which reconstructs a videohologram with the help of sub-holograms. This method is also known astiling. Sub-holograms which are encoded on a very fast electronicallyaddressable spatial light modulator (EASLM) are sequentially imaged intoan intermediate plane. This process is executed at a high speed suchthat a observer perceives the reconstructions of all sub-holograms as asingle reconstruction of a 3D object. The sub-holograms are arranged ina matrix structure in the intermediate plane by a specially designedillumination and imaging system, for example including a shutter whichis controlled in synchronism with the EASLM and which only allows thecorresponding sub-hologram to be transmitted and which in particularblocks unused diffraction orders. However, the demands made on thedynamic properties of the SLM used for reconstructing the sub-hologramsare high, and a flat design does not appear to be feasible.

The above-mentioned solutions have the following major disadvantages incommon. The spatial extension of the reconstruction is limited by thesmall size of the light modulators used for hologram reconstruction. Thetiling method described in WO 00/75699 generally allows large scenes tobe reconstructed, but this requires a voluminous design of the device.Because of the large number of pixels used, the computational loadrequired to compute the hologram and the demands made on the datatransfer rate will increase substantially, which makes it ratherdifficult to achieve real-time reconstructions. When using thesequential tiling method, as known from WO 00/75699, great demands aremade on the dynamic properties of the SLM used.

SUMMARY OF THE INVENTION

Now, the object of the present invention is to provide a projectiondevice for the holographic reconstruction of two- and three-dimensionalscenes which eliminates the above-mentioned disadvantages exhibited byprior art solutions and which reconstructs and renders visible scenes ofany size in a large reconstruction space so that large moving scenes canbe reconstructed simply, inexpensively and with high quality using asmall number of optical elements.

The object is solved as regards the projection device aspect of theinvention by the features of the claimed invention and as regards themethod aspect by the features of the claimed invention.

According to the present invention, the object is solved by a projectiondevice for the holographic reconstruction of scenes comprising a lightmodulator, an imaging system with at least two imaging means and anillumination means with at least one light source with sufficientlycoherent light for illuminating a hologram which is encoded on the lightmodulator, where the at least two imaging means are arranged in relationto each other such that a first imaging means images the light modulatorin an enlarged fashion on to a second imaging means and that the secondimaging means images a plane of a spatial frequency spectrum of thelight modulator into an observer plane, which comprises at least onevirtual observer window, where the virtual observer window correspondswith a diffraction order of the spatial frequency spectrum.

According to the invention, the projection device comprises in additionto the light modulator and illumination device for emitting sufficientlycoherent light an imaging system which comprises the first and thesecond imaging means. The light modulator is a spatial light modulatorof small size and will therefore be referred to as micro SLM below. Themicro SLM is imaged in an enlarged fashion by the first imaging means onto the second imaging means, whereupon the spatial frequency spectrum(Fourier spectrum) of the micro SLM is imaged by the second imagingmeans into the virtual observer window. The observer window is thusrepresented by the image of the used diffraction order of the Fourierplane of the hologram. In order for the first imaging means to be ableto image the entire micro SLM on to the second imaging means, allcontributions of a desired diffraction order must be covered by thefirst imaging means. This is achieved by focusing the light which ismodulated by the micro SLM in the plane of the first imaging means, inwhich the spatial frequency spectrum is created. For this, the micro SLMcan be illuminated by a wave, which converges behind the micro SLM, seenin the direction of light propagation. Consequently, both the Fourierplane of the micro SLM and the first imaging means are situated in theplane of the spatial frequency spectrum. A frustum-shaped reconstructionspace is defined by the second imaging means in combination with theobserver window. In this frustum, a reconstructed scene, preferably areconstructed three-dimensional scene, is presented to one or multipleobservers. The reconstruction space also continues backwards to anyextent beyond the second imaging means. The observer can thus watch thereconstructed scene in the large reconstruction space through theobserver window. In this document, the term ‘sufficiently coherentlight’ denotes light which is capable of generating interference for thereconstruction of a three-dimensional scene.

Such a projection device according to this invention thus only containsa small number of optical elements used for holographic reconstruction.Compared with known optical devices, only little demands are made on thequality of the optical elements. This ensures an inexpensive, simple andcompact design of the projection device, while light modulators of smallsize may be used, such as micro SLM previously used in other projectiondevices. The limited size of the micro SLM also restricts the number ofpixels. This reduces considerably the time needed to compute thehologram, which in turn allows commercially available computer equipmentto be used.

In a preferred embodiment of the invention a spatial frequency filtercan be provided in the plane in which the spatial frequency spectrum ofthe light modulator exists.

One- or two-dimensional holograms which are encoded on micro SLM inpixels, where the pixels are arranged in a regular pattern, create aperiodic continuation of the spatial frequency spectrum in the Fourierplane. In order to suppress or eliminate the periodicity, a spatialfrequency filter, here in particular an aperture, which only transmitsthe used diffraction order can preferably be disposed in this plane. Theindividual diffraction orders are typically overlapped, so that theaperture either cuts off information or lets pass unwanted information.However, the individual diffraction orders can be separated by way oflow pass filtering of the information shown on the micro SLM, so thatthe information is no longer cut off by the aperture. The aperture canbe generalised as a spatial frequency filter which filters out thedesired diffraction order, which blocks quantification errors or othererror of the micro SLM, or which modulates the wave field in anothersuitable way, for example for compensating aberrations of the projectiondevice. This is done, for example, in that the spatial frequency filteradds the function of an aspherical lens.

Another advantage is that the reduction of the spatial frequencyspectrum to one diffraction order and the image of that diffractionorder and of the aperture as an observer window prevents anycross-talking, which would typically occur in reconstructions when usinglight modulators with a matrix structure. This allows to serve a lefteye and a right eye of an observer one after another in a multiplexprocess without cross-talking. Moreover, a multiplex process with theaim to serve multiple persons only then becomes possible.

With light modulators which do not exhibit a regular pixel structure,i.e. which do not cause sampling, the Fourier plane does not showperiodicity either. An aperture thus becomes superfluous. Such lightmodulators are, for example, OASLM.

Another preferred embodiment can for the generation of the spatialfrequency spectrum comprise a third imaging means, which is disposednear the light modulator.

The third imaging means generates in its image-side focal plane thespatial frequency spectrum as the Fourier transform of the hologramencoded on the micro SLM. Using a third imaging means is particularlypreferable in conjunction with collimated illumination, because withoutthis imaging means the light would only reach the first imaging means ata large diffraction angle. The third imaging means can be disposed forexample in front of or behind the micro SLM. Consequently, the thirdimaging means focuses into its image-side focal plane the light or waveemitted by the micro SLM. However, it is also possible that a slightlyconverging wave is emitted by the micro SLM and that its focusing isstrengthened by using a further imaging means. However, the thirdimaging means is not necessary if a converging wave is used forillumination, because the reconstruction wave incident on the micro SLMcan preferably be adjusted such that it converges about in the plane ofthe first imaging means. In any case, a focal plane is always created,which represents the Fourier plane of the micro SLM, in which also thefirst imaging means is disposed.

In order to make available the observer windows for the observer(s) in alarge region, a position detection system can be used to detect the eyepositions of the at least one observer while viewing the reconstructedscene.

The position detection system detects the eye positions or pupilpositions of the observer(s) who are viewing the reconstructed scene.The scene is encoded in accordance with the observer's eye position.Then, the observer window can be tracked according to the new eyeposition. In particular, representations fixed in space but withrealistic change in perspective, and representations with exaggeratedchange in perspective are possible. The latter is defined as a type ofrepresentation where the change in angle and position of the scene isgreater than the change in angle and position of the observer.

At least one deflection element is provided in the display device inorder to track the at least one observer window according to theobserver's eye position. Such deflection elements may be mechanical,electric or optical elements.

The deflection element can for example be disposed in the plane of thefirst imaging means in the form of a controllable optical element, whichvirtually shifts the spectrum like a prism. However, it is also possibleto provide a deflection element near the second imaging means. Thisdeflection element then has the effect of a prism and, optionally, theeffect of a lens. Thereby, the observer window is tracked laterally and,optionally, axially. This arrangement of the deflection element near thesecond imaging means is particularly preferable, because the entireimaging system from the light source to the second imaging means is thena static system. This means that the optical path up to the secondimaging means will always be constant. First, this minimises the demandsmade on that section of the optical system, because the entry pupil ofthe first and second imaging means can be kept at a minimum. If themicro SLM or its image was to be displaced in order to track theobserver window, the entry pupil of the first and second imaging meansalways had to be larger. This substantially reduces the demands made onthe second imaging means. Secondly, the imaging properties of thatstatic section of the optical system can be corrected optimally.Thirdly, the image of the micro SLM does not move on the second imagingmeans. This makes for example the position of the reconstruction of atwo-dimensional scene on the second imaging means independent of theobserver position.

The object is further solved according to the invention by a method forthe holographic reconstruction of scenes where an imaging system with atleast two imaging means images sufficiently coherent light of anillumination device with at least one light source into an observerplane, where the at least one light source illuminates a light modulatorwhich is encoded with a hologram, where in a first step a spatialfrequency spectrum is generated as a Fourier transform of the encodedhologram in a plane of a first imaging means, whereupon in a second stepthe first imaging means images the light modulator into a plane of asecond imaging means, where the second imaging means images the spatialfrequency spectrum from the plane of the first imaging means into atleast one virtual observer window in the observer plane, whereby areconstructed scene is presented to at least one observer in an enlargedfashion in a reconstruction space which stretches between the secondimaging means and the virtual observer window, where the size of thereconstruction space is expanded due to the enlarged image of the lightmodulator.

According to the invention, for reconstructing the scene using acoherent or partially coherent illumination, in a first step the spatialfrequency spectrum is created as the Fourier transform of the hologramencoded on the light modulator, here on a micro SLM, in the plane of thefirst imaging means. In a second step, the image of the micro SLM isthen imaged by the first imaging means into a plane on to the secondimaging means, whereby the micro SLM is enlarged. In a second step, theimage of the micro SLM is then imaged by the first imaging means into aplane on to the second imaging means, whereby the micro SLM is enlarged.Following the enlarged image of the micro SLM, the image of the spatialfrequency spectrum is imaged in a third step by the second imaging meansfrom the plane of the first imaging means into the observer plane, thusforming a virtual observer window in the observer plane. Thereconstruction space, which stretches from the observer window to thesecond imaging means, and in which the reconstructed scene is providedin an enlarged fashion to one or multiple observers, is also enlargedaccordingly. It must be noted that the reconstruction space is notlimited by the second imaging means and the observer window, but itcontinues backwards beyond the second imaging means.

With the help of the method according to this invention, two- and/orthree-dimensional scenes can be represented in an enlarged fashionsimultaneously or one after another, at high quality and in an enlargedreconstruction space for viewing. In mixed 2D/3D representations, theplane of the 2D representation is preferably laid inside thethree-dimensional scene. In a 2D only representation, the plane of the2D representation can preferably be laid in the second imaging means.The enlarged image of the micro SLM will then appear in this plane, whenthe micro SLM is in this case encoded with the two-dimensional image.The two-dimensional image can also preferably be moved towards or awayfrom the observer.

According to a preferred embodiment of the method it may be providedthat aberrations of the imaging means are taken into account whencomputing the hologram and compensated by the light modulator.

Aberrations result in discontinuities in the frequency spectrum and inthe images, said discontinuities adversely affecting the quality of thereconstructions. When positioning the first imaging means in the Fourierplane of the micro SLM, thanks to the focusing, the first imaging meansfor the image only has a minimal lateral extent. This ensuresaberrations of the first imaging means to be minimised. Further, it mustbe ensured that the first imaging means images the micro SLM in anenlarge fashion, completely and homogeneously illuminated on to thesecond imaging means. Aberrations of the second and, if applicable,further imaging means can be compensated by the micro SLM. Phase errorswhich occur in conjunction with aberrations can be corrected easily byan additional according phase shift.

It is further possible that a spatial frequency filter compensatesaberrations of the imaging means used in the projection device.

Further embodiments of the invention are defined by the other dependentclaims. Embodiments of the present invention will be explained in detailbelow and illustrated in conjunction with the accompanying drawings. Theprinciple of the invention will be explained based on a holographicreconstruction with monochromatic light. However, it appears to thoseskilled in the art that this invention may as well be applied to colourholographic reconstructions, as indicated in the description of theembodiment.

BRIEF DESCRIPTION OF THE DRAWINGS

Figures:

FIG. 1 shows the working principle of a projection device for theholographic reconstruction of scenes with an imaging system according tothe invention.

FIG. 2 shows a detail of the projection device shown in FIG. 1 duringthe impingement of an oblique plane wave on a light modulator.

FIG. 3 shows a detail of the projection device shown in FIG. 1 duringthe impingement of convergent wave on the light modulator.

FIG. 4 shows another embodiment of the projection device according tothis invention with a reflective light modulator and a beam splitterelement.

FIG. 5 shows a deflection element comprised in the projection device,said element being used for tracking an observer window.

FIG. 6 shows another possibility of tracking the observer window in theprojection device.

FIG. 7 shows another embodiment of the projection device according tothis invention with a concave mirror as the second imaging means.

FIG. 8 shows the projection device shown in FIG. 1 where a singlereconstructed point of the scene is viewed.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 shows the working principle of the projection device according tothis invention, where an imaging system 3 images an illumination device1, here a point light source, to an observer plane 6. The imaging system3 comprises a first imaging means 4 and a second imaging means 5. Thelight source 1 emits coherent or sufficiently coherent light, which isrequired for a holographic reconstruction of a scene. The light source 1can be a laser, LED(s) or other light sources, where colour filters canalso be used.

Now, the working principle of the projection device will be describedwith reference to FIG. 1. A wave emitted by the light source 1 isconverted into a plane wave 7 with the help of a collimator lens L. Thewave 7, which comes from the light source 1, and which is assumed to beplane after its passage through the collimator lens L, hits at a rightangle a transmissive spatial light modulator 8 with regularly arrangedpixels, which represents an encoded dynamic hologram 2, e.g. a CGH,where the wave front of the plane wave 7 is modulated at equidistantpositions in the spatial light modulator 8 so to form a desired wavefront. The spatial light modulator 8 has a small size and will thereforebe referred to as micro SLM below.

A third imaging means 9 is disposed behind the micro SLM 8, seen in thedirection of light propagation. The third imaging means 9, here a lens,can alternatively be disposed in front of the micro SLM 8 if atransmissive light modulator is used. It generates in its image-sidefocal plane 10 a spatial frequency spectrum as a Fourier transform ofthe information encoded on the micro SLM 8 when being illuminated withthe plane wave 7. The spatial frequency spectrum can also be referred toas Fourier spectrum. If the micro SLM 8 is illuminated with non-planeconverging or diverging waves, the focal plane 10 is displaced along anoptical axis 11.

If the micro SLM 8 is illuminated with a plane wave and if the thirdimaging means 9 was omitted in the projection device, only light withaccordingly large diffraction angle could reach the second imaging means5.

The first imaging means 4 is disposed in the immediate vicinity of thefocal plane 10 of the third imaging means 9. This first imaging means 4images the micro SLM in an enlarged fashion into a plane 12, whichcoincides with the second imaging means 5 or is disposed in itsimmediate vicinity. Here, the second imaging means 5 is a lens which ismuch larger than the other imaging means 4 and 9, so that a scene 13which is as large as possible is reconstructed in a frustum-shapedreconstruction space 14. While the micro SLM 8 is imaged into the plane12, its spatial frequency spectrum is at the same time imaged into theobserver plane 6 by the second imaging means 5. A virtual observerwindow 15 is thus formed, which is not physically existing, and whoseextent corresponds with the image of a period of the spatial frequencyspectrum. The observer(s) can watch the reconstructed scene 13 throughthe observer window 15. The reconstruction of the scene 13 is generatedin a frustum-shaped reconstruction space 14, which stretches between theedges of the observer window 15 and the second imaging means 5. Thereconstruction space 14 can also continue backwards to any extent beyondthe second imaging means 5. In other words, the reconstruction space 14is not limited by the second imaging means 5 and the virtual observerwindow 15, but it can continue backwards beyond the second imaging means5.

Due to the equidistant scanning of information on the micro SLM 8, whichis assumed to represent a regular matrix, this micro SLM 8 createsseveral diffraction orders in periodic continuation in the focal plane10 of the third imaging means 9. This periodic continuation exhibits inthe focal plane 10 a periodicity interval, whose size is reciprocal tothe pitch of the micro SLM 8. The pitch corresponds therein to thedistance between the scanning points on the micro SLM 8. The secondimaging means 5 images the periodic distribution in the focal plane 10into the observer plane 6. If an observer stays within a diffractionorder in the observer plane 6, he would see an undisturbed reconstructedscene 13 with one eye, but the other eye may perceive disturbing higherdiffraction orders at the same time.

For spatial light modulators which are organised in a matrix and whichhave a low resolution, namely a pixel pitch>>λ (reconstructionwavelength), the periodicity angle can be expressed in adequateapproximation by (λ/pitch). Assuming a wavelength λ of 500 nm and apitch of the micro SLM 8 of 10 μm, a diffraction angle of about ±1/20rad would be achieved. If the third imaging means 9 has a focal distanceof 20 mm, this angle corresponds to a lateral extent of the periodicityinterval of about 1 mm.

In order to suppress the periodicity, an aperture 16 is disposed in thefocal plane 10 behind the first imaging means 4, said aperture 16 onlytransmitting one periodicity interval or only the desired diffractionorder. The aperture has the effect of a low-pass, high-pass andband-pass filter in this case. The aperture 16 is imaged by the secondimaging means 5 into the observer plane 6, where it forms the observerwindow 15. The benefit of an aperture 16 in the projection device isthat cross-talking of further periods to the other eye or to eyes ofanother observer is prevented. However, a condition for this is abandwidth-limited spatial frequency spectrum of the micro SLM 8.

Spatial light modulators which do not exhibit periodicity in the focalplane 10, such as optically addressable light modulators (OASLM), do notrequire an aperture 16 to be used.

Spatial light modulators are often organised in a matrix. The spatialfrequency spectrum in the focal plane 10 will thus be continuedperiodically. However, the three-dimensional scene will typicallyrequire the hologram 2 to be encoded on the micro SLM 8 whose spatialfrequency spectrum is larger than the periodicity interval in the focalplane 10. This results in an overlapping of individual diffractionorders. The aperture 16 in this focal plane 10 would in this case cutoff an information-carrying part of the used diffraction order on theone hand and let pass higher diffraction orders on the other. In orderto suppress such effects, the three-dimensional scene can be limited inthe spatial frequency spectrum of the focal plane 10 by precedingfiltering. Preceding filtering or bandwidth limitation is alreadyconsidered when computing the hologram 2. The bandwidth-limiteddiffraction orders are thus separated from each other. The aperture 16in the focal plane 10 then blocks off the higher diffraction orderswithout limiting the selected diffraction order. This prevents theinformation for one eye from cross-talking to the other eye of theobserver or to other observers.

The aperture 16 can also be extended so to form a spatial frequencyfilter. The spatial frequency filter is a complex-valued modulationelement, which modifies the amplitude and/or phase of the incident wave.The spatial frequency filter thus also serves other functions besidesseparating the diffraction orders, it suppresses for example aberrationsof the third imaging means 9.

To be able to track the observer window 15 according to the movement ofthe eyes of the observer(s), the projection device comprises a positiondetection system 17 which detects the actual position of the observereyes while the observer(s) watch the reconstructed scene 13. Thisinformation is used for tracking the observer window 15 using suitablemeans. The encoding of the hologram 2 on the micro SLM 8 can thus beadapted to the actual eye position. The reconstructed scene 13 isthereby re-encoded such that it appears horizontally and/or verticallydisplaced and/or turned by an angle, according to the actual observerposition. In particular, representations fixed in space but withrealistic change in perspective and representations with exaggeratedchange in perspective are possible. The latter is defined as a type ofrepresentation where the change in angle and position of the object isgreater than the change in angle and position of the observer. Theprojection device comprises a deflection element (not shown in FIG. 1),which is shown in more detail in FIG. 5, for tracking the observerwindow 15 according to the eye positions.

In the case of a low resolution of the micro SLM 8, the observer window15 does not permit the observer to watch the reconstructed scene 13simultaneously with both eyes. The other eye of the observer can beaddressed sequentially in another observer window, or simultaneouslyusing a second optical path. If the resolution of the micro SLM 8 issufficiently high, the holograms for the right eye and for the left eyecan be encoded on one micro SLM, using spatial multiplexing methods.

When using one-dimensional spatial light modulators, it will only bepossible for a one-dimensional reconstruction to take place. If theone-dimensional spatial light modulator is oriented vertically, thereconstruction will only be vertical too. With these vertically encodedholograms, the spatial frequency spectrum of the spatial light modulatoronly shows a periodic continuation in vertical direction in the focalplane 10. The light wave leaving the one-dimensional spatial lightmodulator propagates accordingly in horizontal direction. When usingone-dimensional spatial light modulators, additional focusing opticalelements, e.g. cylindrical lenses, must be used for focusingperpendicular to the reconstruction direction.

FIG. 2 shows a detail of the projection device shown in FIG. 1.Specifically, this detail shows the micro SLM 8 with the imaging means 4and 9 and the aperture 16. Instead of a plane wave 7 which hits themicro SLM 8 at a right angle, as shown in FIG. 1, an oblique plane wavefront 18 is used in this embodiment. This is particularly beneficial ifthe detour phase encoding method is used to encode the hologram 2.During detour phase encoding, that is when using a pure amplitudehologram, the oblique wave hits adjacent pixels with the requiredphases. If the angle of incidence is chosen accordingly, the phases ofevery third pixel are identical, for example (Burckhardt encoding).Three pixels then encode one complex value. When using the detour phaseencoding method, all diffraction orders are blocked except the typicallyused 1st or −1st diffraction order.

If this is the case, the centre of the zeroth diffraction order in thefocal plane 10 is displaced perpendicular to the optical axis 11, asindicated by the marginal rays shown as broken lines in the Figure. Thefirst imaging means 4 and the aperture 16 are arranged such that the1^(st) or −1^(st) diffraction order is transmitted, as indicated by themarginal rays shown as solid lines.

FIG. 3 also shows a detail of the projection device of FIG. 1. Insteadof a plane wave which hits the hologram at a right angle, a convergingwave 19 is used for the reconstruction. As can be seen in the Figure,the third imaging means 9 can be omitted in the case of convergentillumination, if the converging wave 19 is adjusted such that the firstimaging means 4 is disposed in the focus of the converging wave 19 andthat the spatial frequency spectrum of the hologram 2 encoded on themicro SLM 8 is created in the focal plane 10. If the convergence of theincident wave changes, the point of convergence will move along theoptical axis 11.

FIG. 4 shows another embodiment of the projection device according tothis invention with a reflective micro SLM 8 and a beam splitter element20. The beam splitter element 20 is disposed between the third imagingmeans 9 and the first imaging means 4 and serves to guide the beam ofthe incident plane wave 7. The beam splitter element 20 can be a simpleor dichroic splitter cube, a semipermeable mirror or any other beamcoupler means.

Because the micro SLM 8 in this embodiment is a reflective micro SLM andthe light must thus cover twice the distance because of the reflection,the encoding of the hologram 2 must be adapted accordingly. Injectingthe light wave 7 through a dichroic beam splitter is particularlybeneficial if the three primary colours RGB (red, green, blue) of thescene 13 are reconstructed sequentially. The three light sources for theindividual primary colours are not shown in this embodiment. The sceneis reconstructed as described with reference to FIG. 1. The particularbenefit of the sequential reconstruction is that the optical path isalways identical. Only the encoding must be adapted to thereconstruction at the different wavelengths λ.

This embodiment can be developed further in that separate channels areprovided for each of the three primary colours RGB, each of saidchannels comprising a light source emitting light of one primary colour,a micro SLM 8, imaging means 4 and 9 and an aperture 16 or a spatialfrequency filter. Again, the third imaging means 9 can be omitted if themicro SLM is illuminated using converging waves. Further, beam splitterelements can be used to combine the three channels. For simultaneouscolour reconstruction of the scene 13, a beam splitter element can beprovided which is made up of four adjoined individual prisms betweenwhich there are dichroic layers which exhibit different,wavelength-specific transmittance and reflectance. The light of thethree channels serving the individual primary colours is injectedthrough three side faces, and the superimposed light is emitted throughthe fourth side face. The light which is composed of the three primarycolours then proceeds to the second imaging means 5 so to reconstructthe colour scene.

Parallel arrangements of the three channels are also possible. Thesecond imaging means 5 can therein be used commonly for all threechannels. This way, the scene is simultaneously reconstructed in allthree colours.

Further, it is possible to provide separate channels for each observereye. Again, each channel contains a monochromatic light source of oneprimary colour, a micro SLM 8, imaging means 4 and 9 and an aperture 16.The second imaging means 5 can again be used commonly for the twochannels. The two channels image their observer windows on to theobserver eyes.

Further, it is possible to provide separate channels for each observereye, where each channel comprises three sub-channels for the threeprimary colours RGB.

In all above-mentioned options of colour reconstructions it must be madesure that the reconstructions in the three primary colours are fullycongruent.

The above-mentioned embodiments also allow the observer window 15 to betracked according to the observer eye position, should the observermove. FIG. 5 shows the working principle of a method for tracking theobserver window 15. In order to be able to track the observer window 15in the observer plane 6 as indicated by an arrow in the Figure, thelight beams are deflected by a deflection element 21, here representedby a polygonal mirror, behind the focal plane 10. This way, the observerwindow 15 is tracked to the observer. Mechanical deflection elements,such as polygonal mirrors, galvanometer mirrors and prisms, or opticaldeflection elements, such as controllable grids or other diffractionelements, may be used as deflection elements 21.

The observer window 15 is particularly preferably tracked as shown inFIG. 6. Here, the deflection element 21 has the function of acontrollable prism. The deflection element 21 is disposed near theimaging means 5, i.e. in front of or behind it, seen in the direction oflight propagation, or it forms an integral part of the imaging means 5itself. This deflection element 21 optionally exhibits the effect of alens, in addition to the effect of a prism. Thereby, lateral and,optionally, axial tracking of the observer window 15 is achieved.

Such a deflection element 21 with prism function can for example bemanufactured by embedding prismatic elements which are filled withbirefringent liquid crystals in a substrate made of transparentmaterial, or by surrounding those elements with a substrate whichexhibits a refractive index different from that of the prismaticelements. The angle by which a light beam is deflected by one of thoseelements depends on the ratio of the refractive indices of the substratematerial and liquid crystal. The orientation of the liquid crystals andthus the effective refractive index is controlled by an electric fieldto which those elements are exposed. This way the deflection angle canbe controlled with the help of an electric field, thus tracking theobserver window 15 according to the movements of the observer.

It is further possible to displace the light source 1 perpendicular tothe optical axis 1 in order to track the observer window 15. For this,the first imaging means 4 and the aperture 16 must be displacedaccording to the new position of the focal point in the focal plane 10.Again, the zeroth diffraction order of the micro SLM 8 is then situatedaround the focal point in the focal plane 10.

FIG. 7 shows another embodiment of the projection device according tothis invention with a concave mirror 22, instead of the lens shown inFIG. 1, as the second imaging means 5. The scene is reconstructed in thesame way as described in conjunction with FIG. 1. However, here thefirst imaging means 4 does not image the micro SLM 8 into the plane 12,but into a plane 23 on to the concave mirror 22 or to its immediatevicinity. Because the wave is reflected by the concave mirror 22, theobserver window 15 is formed according to this reflection. Accordingly,the reconstruction space 14, in which the reconstructed scene 13 can beviewed, stretches between the observer window 15 and the concave mirror22. As already mentioned above, the reconstruction space 14 can alsocontinue backwards to any extent beyond the concave mirror 22. This way,a more compact projection device can be provided. Further advantages ofusing a concave mirror 22 are that in contrast to a lens, it can be madefree of aberrations more easily, its manufacturing process is simpler,and it weighs less.

It is particularly beneficial to use a flat focusing mirror as theimaging means 5. This imaging means 5 can be a holographic opticalelement (HOE) or a diffractive optical element (DOE). The imaging means5 exhibits a phase pattern which lets the reconstruction wave convergeinto the observer window 15 after the reflection. The imaging means 5 inthe form of a HOE or DOE thus fulfills the same function as the concavemirror 22. The advantages of a HOE or DOE are that it is of a flatdesign and that it can be manufactured inexpensively. Such mirrors canbe made using known methods, e.g. interferometry or lithography, byembossing, forming and subsequent curing, extruding or in any other way.They consist of photo or resist material, polymers, metal, glass orother substrates. They can also exhibit reflective layers on a relief.

FIG. 8 shows the projection device of FIG. 1 with a single reconstructedpoint 24 of the scene 13. The imaging means 5 is relatively largecompared with the two imaging means 4 and 9. Only small sections of itmust be free of aberrations. To facilitate understanding, only onereconstructed point 24 of the scene 13 will be discussed, while theentire scene of course comprises a multitude of points. The point 24 isonly visible within the observer window 15. The observer window 15 is animage of the selected diffraction order from the plane 10 and serves asa window through which the observer can watch the reconstructed scene13. Bandwidth-limited encoding of the hologram 2, in order to preventoverlapping from higher diffraction orders, has already been describedabove. This encoding ensures that the diffraction orders do not overlapin the plane 10. The same holds true for the image in the observer plane6. Each individual point of the reconstructed scene 13 is only generatedby a part of the micro SLM 8 on the second imaging means 5. Theprojection of the marginal rays of the observer window 15 through thepoint 24 on to the second imaging means 5 clearly shows a small regionon the imaging means 5 which contributes to the reconstruction of thatpoint 24. This means that for each individual point of the scene thereis such a limited region on the imaging means 5. These regions are smallin comparison with the large second imaging means 5. The requirementsfor coherence thus relate to those small regions only, in particularcompliance with the requirement for sufficiently small wave frontdistortion<<λ/10. The image must only be of highly-coherent quality inthose small regions, where all points of the scene 13 must beconsidered. It is thus not necessary for the imaging means 5 to exhibitan extremely low wave front distortion across the entire element. Thisreduces the demands made on the second imaging means 5 largely on togeometrical form stability.

Further, the projection device does not only take advantage of the microSLM 8 for the reconstruction of very large two- and three-dimensionalscenes 13 which are formed in the reconstruction space 14 through theobserver window 15, but preferably also uses it simultaneously forcorrections to the optical imaging means 4, 5 and 9. Aberration-freeimaging means should be used for holographic reconstructions. Examplesof corrections of aberrations will be described below. Aberrations ofthe third imaging means 9 become apparent as phase errors by which thewave front deviates from the ideal wave front. In a hologram withoutencoded information, where a plane wave leaves the micro SLM 8, thediffraction-limited wave should be focused in the plane 10, in which thefirst imaging means 4 and a spatial frequency filter 16—as an aperturefor suppressing undesired diffraction orders and for fulfilling otherfunctions, such as aberration correction—are disposed.

However, aberrations cause this above-mentioned focus to be blurred andthus disturbances in the spatial frequency spectrum to occur whichadversely affect the quality of the reconstruction. Such phase errorscan be compensated easily by an additional phase shift. Another meansfor correcting the third imaging means 9 has already been described inconjunction with the function of the spatial frequency filter.

The enlarged image of the micro SLM 8 by the first imaging means 4 on tothe second imaging means 9 is typically prone to aberrations. Enlargingoptical systems for the imaging means 4 are for example opticalprojection systems as used in back-projection TV sets which arecommercially available today. Image definition is a major criterion, sothat chiefly spherical aberrations, but also coma and astigmatism arealready widely suppressed in these optical systems. While residualdistortion and field curvature in the projection are tolerable for theuser of those devices, such aberrations may cause the reconstructions tobe greatly biased if they occur in the present holographic projectiondevice. The distortion of the first imaging means 4 means a lateralgeometrical deviation of the enlarged image of the micro SLM 8 on to theimaging means 5. The waves which leave the second imaging means 5 dothen not converge in the desired position of the reconstructed objectpoint, but are shifted.

A major optical error is the field curvature when imaging the micro SLM8 on to the second imaging means 5. Field curvature means mainly thatthe required phase values are biased on the imaging means 5, whichbecomes apparent in the form of a three-dimensional distortion, i.e.lateral and axial. Both effects, field curvature and distortion, as wellas coma and astigmatism can generally be kept sufficiently small by wayof careful design and low manufacturing tolerances of the first imagingmeans 4; however, this requires great efforts and is rather costly.Phase bias due to field curvature in the projection device can becompensated preferably by the micro SLM 8. Such phase errors can becompensated by an additional phase shift. Moreover, coma and astigmatismcan also be reduced by appropriate encoding. The distortion can forexample be compensated by selecting other pixels of the micro SLM 8,i.e. by encoding the hologram values on to pixel positions which weredetermined taking into consideration the extent of distortion. In asimilar way, i.e. as described for the first imaging means 4, theaberrations of the second imaging means 5 are also compensated with thehelp of the micro SLM 8. The deviations of the waves which leave thesecond imaging means 5 must typically be much smaller than λ/10. Thisrequires enormous efforts again. Using the above-mentioned possibilityof correction, aberrations with regard to the second imaging means 5 canalso be corrected easily by way of according encoding.

Generally, all and any aberrations of the imaging means 4, 5 and 9 canbe reduced or compensated with the help of the micro SLM 8. Theaberrations are determined in a suitable way before the reconstruction.Thus computed phase errors can be compensated by an additional phaseshift of the micro SLM 8.

The present projection device makes it possible for spatial lightmodulators of small size to be used for the reconstruction and viewingof large, two- or three-dimensional scenes. The observer(s) can thusmove freely in the observer plane 6 while they watch a reconstructedscene. Two- and three-dimensional scenes can be shown simultaneously orone after another. Moreover, the projection device consists ofcommercially available optical elements with relatively low demands asregards manufacturing precision and freedom of aberrations. First, theimaging means 4 and 5 can be corrected by the micro SLM 8, and secondly,a low wave front distortion is only required across small regions of thelarge imaging means 5.

In the special case of a mere two-dimensional image, as in theapplication of today's television, the image is projected on to theimaging means 5 or into its immediate vicinity. The hologram 2 iscomputed such that a two-dimensional scene is reconstructed in the plane12 or 23 of the second imaging means 5. In addition, the observer who iswatching the scene can axially displace a plane in which areconstruction of the two-dimensional scene is provided by way ofre-computing the hologram 2. This means that the representation can bemoved towards or away from the observer. Moreover, details can be zoomedso that the observer can watch those more precisely. These activitiescan be initiated by the respective observer himself interactively.

Possible applications of the holographic projection device includedisplays for a two- and/or three-dimensional presentation in private orworking environments, for example computer displays, TV screens,electronic games, in the automotive industry for displaying information,in the entertainment industry, in medical engineering, here inparticular for minimally-invasive surgery applications or spatialrepresentation of tomographically established information, and inmilitary engineering for the representation of surface profiles. Itappears to those skilled in the art that the present projection devicecan also be applied in other areas not mentioned above.

The invention claimed is:
 1. A method for the holographic reconstructionof scenes, comprising: imaging sufficiently coherent light of anillumination device having at least one light source into an observerplane by an imaging system having at least two imaging means,illuminating a light modulator encoded with a hologram by the at leastone light source, generating a spatial frequency spectrum as a Fouriertransform of the encoded hologram in a plane of a first imaging means,imaging the light modulator into a plane of a second imaging means bythe first imaging means, imaging the spatial frequency spectrum from theplane of the first imaging means into at least one virtual observerwindow in the observer plane by the second imaging means, and presentinga reconstructed scene to at least one observer in an enlarged fashion ina reconstruction space, where the size of the reconstruction space isexpanded due to the enlarged imaging of the light modulator.
 2. Themethod according to claim 1, wherein the second imaging means images thespatial frequency spectrum into the virtual observer window, throughwhich an observer watches the reconstructed scene.
 3. The methodaccording to claim 1, wherein a third imaging means is disposed near thelight modulator, said third imaging means generating in its image-sidefocal plane the spatial frequency spectrum of the hologram encoded onthe light modulator.
 4. The method according to claim 1, wherein aspatial frequency filter provided in the imaging system lets pass aselected periodicity interval of the spatial frequency spectrum.
 5. Themethod according to claim 4, wherein the spatial frequency filtercompensates aberrations of the imaging means.
 6. The method according toclaim 1, wherein aberrations of the imaging means are taken into accountwhen computing the hologram and compensated by the light modulator. 7.The method according to claim 1, wherein the bandwidth of the spatialfrequency spectrum in the plane of the first imaging means is limitedduring a computation of the hologram.
 8. The method according to claim1, wherein a position detection system detects eye positions of at leastone observer who watches the reconstructed scene.
 9. The methodaccording to claim 8, wherein the holographic encoding of the lightmodulator is updated during changing of the eye position of theobserver.
 10. The method according to claim 9, wherein the reconstructedscene is encoded such that it appears horizontally and/or verticallydisplaced and/or turned by an angle in accordance with the changing ofthe eye position of the observer.
 11. The method according to claim 8,wherein at least one virtual observer window in the observer plane istracked in accordance with the eye position of the observer.
 12. Themethod according to claim 11, wherein at least one deflection elementtracks the at least one virtual observer window.
 13. The methodaccording to claim 1, wherein the hologram is computed such that atwo-dimensional scene is reconstructed in the plane of the secondimaging means.
 14. The method according to claim 1, wherein the observerwho is watching the scene axially displaces a plane in which areconstruction of a two-dimensional scene is provided by way ofre-computing the hologram.
 15. The method according to claim 1, whereina colour reconstruction of the scene is performed sequentially for thethree primary colours.
 16. The method according to claim 1, wherein acolour reconstruction of the scene is performed simultaneously for thethree primary colours.
 17. The method according to claim 16, wherein thesimultaneous colour reconstruction of the scene is performed using threeparallel channels, each of which comprising a light source, a lightmodulator, a first imaging means and a third imaging means.